1. Field of the Invention
The present invention relates to a method for crystallizing an amorphous silicon thin-film for use in a thin-film transistor (TFT) and a thermal annealing apparatus therefor, and more particularly, to a method for crystallizing an amorphous silicon thin-film for a TFT in which amorphous silicon thin-films on a large-area glass substrate for use in a TFT-LCD (TFT-Liquid Crystal Display) are crystallized uniformly and quickly by a scanning method using a linear lamp to prevent deforming of the glass substrate, and a method for fabricating a polycrystalline TFT using the same, and a thermal annealing apparatus therefor.
2. Description of the Related Art
To enhance driving speed and resolution and improve productivity through integration of driving circuits, replacement of amorphous silicon TFT by polycrystalline silicon TFT is being vividly performed under study. Difficulties confronting when fabricating a polycrystalline silicon thin-film are to prevent a deform of glass which is used as a substrate. To do so, amorphous silicon should be crystallized within a temperature and time at which the glass substrate is resistant without being deformed.
A metal-induced lateral crystallization (MILC) method proposed to overcome the above difficulties can lower an amorphous silicon crystallization temperature at 500xc2x0 C. or below and has advantages using simple equipment and processes compared with other crystallization methods. In this MILC method, a metal thin-film such as Ni, Pd and so on, is partially formed on the interface between the surface of an amorphous silicon thin-film and a substrate, and is thermally annealed at 500xc2x0 C. or so, in such a manner that crystallization proceeds at the portion where the metal thin-film has been formed and in lateral direction thereof. A polycrystalline silicon TFT can be fabricated using the above MILC, in which a device having an excellent electrical characteristic can be fabricated at 500xc2x0 C. or below.
FIG. 1 is a sectional view showing a manufacturing process of a TFT using the MILC method. As shown, an amorphous silicon thin-film 10 is formed in the form of an island on the whole surface of a glass substrate 100. Then, a gate insulation film 12 and a gate electrode 13 are formed in turn. Then, a metal film 14 of Ni is deposited on the whole surface of the substrate including a source region 10S and a drain region 10D and then annealed, to thereby crystallize a channel region 10C of the amorphous silicon thin-film 10 by the MILC method.
The above method has a shorter thermal processing time than that of a method for forming a gate electrode after depositing and crystallizing an amorphous silicon thin-film on the whole surface of a substrate. Since the above method crystallizes only a channel region, yield is considerably improved.
To crystallize an amorphous silicon by thermal annealing at the state where the metal thin-film is not formed requires thermal processing of about 30 hours at a temperature of 600xc2x0 C. or above. Meanwhile, the above MILC technique shows a crystallization velocity of 1.6 xcexcm/hr or more at 500xe2x96xa1xe2x96xa1 so, which must be very useful crystallization method. In the MILC method, when a thermal annealing temperature is 600xc2x0 C. or above, the lateral crystallization proceeds more quickly depending upon the temperature. Thus, the lateral portions of the portion where the metal thin-film has been formed are crystallized all by the MILC method.
Meanwhile, in the case of a next generation large-area glass substrate, it does not facilitate to implement a furnace thermal annealing apparatus and it is difficult to enhance productivity because of a long-term thermal annealing time. For this reason, a thermal annealing apparatus adopting a number of lamps shown in FIG. 2 has been proposed.
FIG. 2 is a sectional view schematically showing a lamp thermal annealing apparatus which is used for crystallization of an amorphous silicon thin-film according to the conventional prior art. As depicted, a bottom layer oxide film 22 is formed on a substrate 21. A Ni metal layer 24 is formed on the surface of an amorphous silicon thin-film 23 formed on the oxide film 22. Then, a process for thermally processing the amorphous silicon thin-film at high temperature for a second using lamps 29 and cooling it for five seconds is performed at least once, to thereby crystallize the amorphous silicon thin-film by the MILC method.
In the MILC method, only an opaque amorphous silicon thin-film is heated and crystallized and a transparent glass substrate is not heated by the lamps, to accordingly prevent a deformation of the glass substrate. The reason for cooling the amorphous silicon for five seconds or so is to block the heats of the heated amorphous silicon from being transferred to the glass substrate, in order to prevent deforming of the glass substrate due to the heats transferred from the amorphous silicon to the glass substrate.
However, the above method for heating the whole surface of the substrate is also limited to implement a thermal annealing apparatus for uniformly heating a large-area glass substrate, such as a substrate of 600 mmxc3x97500 mm or larger. As described above, if all the portions of the substrate are not uniformly heated, a thermal processing time should be longer in order to crystallize all the portions of the substrate. Thus, the temperature of the amorphous silicon may be locally raised, so that the substrate may be deformed.
To solve the above problems, it is an object of the present invention to provide an amorphous silicon crystallization method capable of crystallizing an amorphous silicon without deforming a large-area transparent glass substrate irrespective of the size of the substrate, employing a continuous process rapid thermal annealing (RTA) method using light.
It is another object of the present invention to provide a method for manufacturing a low-temperature polycrystalline silicon thin-film transistor capable of greatly improving a crystallization uniformity and a crystallization velocity by employing both a continuous process rapid thermal annealing (RTA) method and a metal-induced lateral crystallization (MILC) method simultaneously.
It is still another object of the present invention to provide a thermal annealing apparatus which is used for crystallization of an amorphous silicon thin-film for use in a thin-film transistor (TFT), capable of preventing deformation of a glass substrate, in which an amorphous silicon thin-film is uniformly and rapidly crystallized on a large-are glass substrate for use in a thin-film transistor-liquid crystal display (TFT-LCD) by a continuous process or scanning method using a linear lamp.
To accomplish the above object of the present invention, according to one aspect of the present invention, there is provided a thermal annealing apparatus for crystallizing an amorphous silicon thin-film, the thermal annealing apparatus comprising: supporting means for supporting at least one glass substrate on which the amorphous silicon thin-film has been formed; a light source for illuminating a linear light beam to be focused on the glass substrate from the upper portion of the glass substrate; and scanning driver means for relatively moving one of the supporting means and the light source so that the linear light beam can be illuminated on the silicon thin-film according to a scanning method.
According to another aspect of the present invention, there is also provided an amorphous silicon thin-film crystallization method comprising the steps of: forming an amorphous silicon thin-film on a glass substrate; and illuminating a linear light beam on the amorphous silicon thin-film from the upper portion of the glass substrate according to a scanning method.
Also, a method for manufacturing a polycrystalline silicon thin-film transistor employing the above crystallization method, comprising the steps of: forming an amorphous silicon thin-film on a glass substrate; and crystallizing the amorphous silicon of the thin-film transistor according to a scanning method using a linear light beam.
Here, the step of forming the amorphous silicon thin-film transistor comprises the sub-steps of: forming an active layer composed of the amorphous silicon on the glass substrate; forming a gate insulation film and a gate electrode on the active layer in turn; and depositing a metal thin-film on the resultant glass substrate, wherein the amorphous silicon with respect to a channel region positioned in the lower portion of the gate insulation film is crystallized by a metal-induced lateral crystallization (MILC) method using the metal thin-film.
As described above, the thermal annealing apparatus according to the present invention can locally heat the amorphous silicon to be crystallized by a scanning method where the substrate is transported at the state where the linearly focused light of the lamp is illuminated on the glass substrate. As a result, the amorphous silicon can be crystallized without deforming a large-are transparent glass substrate such as a LCD for a TV irrespective of the size of the substrate and without expanding the size of the thermal annealing apparatus on a three-dimensional basis.